I'm surprised a Rust alignment bug is fixed this late. Performance degradation issues due to bad alignment is usually pretty obvious and somewhat easy to notice. I'm assuming vectorized (SSE) code is often generated for 128-bit loads and stores.

Obviously something must have obscured it until now. Anyone knows why?

Edit: So it was an LLVM issue. Even weirder. https://reviews.llvm.org/D86310

Edit2: Ok, it's obvious: if you "fix" something like this you can break a lot of code that relies on specific struct alignment. Ouch.